One of the best uses of docker is when you need libraries that are newer than what is available on the version of OMV you are using. You might also use it if you want the service in a jail so nothing on the host system could accessed (unless you allow it). For things that have a plugin and docker available, I guess it is just personal preference.

I just had the problem, that after reboot, docker ps showed no images anymore.
I went to the Web-Interface and found that it was complaining that it got no response at port 2375. That's a bit odd, as docker listens on port 2376.
So, I corrected that.

This is a problem in two senses:
1) I do not think that anything should be deleted here.
2) rm fails, as the subvolumes are actually btrfs subvolumes. Here, I think the plugin must distinguish between btrfs and aufs as file-system. In case of btrfs it should use btrfs subvolume delete instead.

In any case: I do not know what caused my initial problem (no containers, even in the shell (docker ps))

I'm not sure what is going on, but I'm been having nothing but issues with OMV and docker. I had a previous setup with working docker using OMV image openmediavault_3.0.79-amd64.iso, but at the time I didn't have any storage setup. I wasn't actively using it, so the other day I started to play around with it again, and applied all the OMV updates, etc. And then setup some storage and tried to move the docker base to a shared folder, which ended up totally messing up my OMV install.

So today I started fresh, with OMV openmediavault_3.0.86-amd64.iso, fully updated it. Installed OMV3-extras, installed Union FS, Snapraid, setup some shares (default settings - admin R/W/X, users R/W/X, rest others R/X). I then enabled Docker in the OMV3-extras plugin. Then installed the Docker UI plugin. Went to the docker UI plugin, set the base path to my docker share, clicked enable and the following happened:

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Docker UI label still present on left side of screen after uninstall and reboot, even though the plugin page shows it as being uninstalled
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If anyone can help it would be appreciated, as I'm trying to evaluate OMV to see if it meets my needs for migrating from my current system... but so far it's been nothing but headaches. I'm not trying to do anything fancy, and haven't messed with out of box settings, etc, but nothing seems to work right...

I'm trying to test a FreePBX docker image except I have no clue how the networking works.

I would advise you to either start the container via CLI or use the macvlan network driver which is supported in the latest version of the plugin (released just a minute ago). This due to the fact that FreePBX uses lots of port ranges, which the plugin does not really support very well. I'm running FreePBX myself within a docker container and I've used the macvlan driver with good results.